This disclosure relates generally to the field of seismic surveying, e.g., marine geophysical surveying, and more specifically to systems and methods for improved performance of marine vibrators, and/or to enable seismic operations in areas where use of conventional air guns and vibrators with high-frequency noise may not be environmentally acceptable.
Seismic sources, including vibrators, are used in geophysical exploration on land and in water-covered areas of the earth. Signals generated by these sources travel downwardly into the subsurface and are reflected from reflecting interfaces in the subsurface. The reflected energy is detected by signal detectors, typically hydrophones or geophones, on or near the earth's surface or near the water surface in water-covered exploration areas.
Most of the acoustic sources presently used in marine seismic acquisition operations are of the impulsive type, in which as much energy as possible is generated during as short a time span as possible. Examples of such impulse sources include air guns and water guns. The frequency content of such sources is typically controllable only to a small degree, and different individual sources are selected and operated together in an array for the generation of different frequency ranges of seismic energy for different seismic surveying needs.
Vibratory acoustic sources, including hydraulically powered sources and sources employing piezoelectric or magnetostrictive material, have been used in marine operations. However, such sources have found only limited use. Although such sources can generate signals over various frequency bands, commonly referred to as “frequency sweeps,” the limited power that such sources known in the art have been able to generate has limited their use in marine operations.
It is well known that as sound waves travel through water and through subsurface geological structures, higher frequency sound waves are typically attenuated more rapidly than lower frequency sound waves, and consequently, lower frequency sound waves can be transmitted over longer distances through water and geological structures than higher frequency sound waves. There has for a long time been a need in the seismic sector of the oil and gas industry for powerful low frequency vibrator type marine seismic energy sources.
It is also important that the spectral content of the seismic energy generated by a vibrator be well known or characterized in order to be able to properly interpret the reflected seismic energy from the subsurface. Control methods used for operating high-powered land-based vibrators are not necessarily adaptable to use in controlling marine vibrators. There also exists a need for a control method for a marine vibrator to assure well characterized energy spectral content.
This specification includes references to “one embodiment” or “an embodiment.” The appearances of the phrases “in one embodiment” or “in an embodiment” do not necessarily refer to the same embodiment. Particular features, structures, or characteristics may be combined in any suitable manner consistent with this disclosure.
Various devices, units, circuits, or other components may be described or claimed as “configured to,” “usable to,” or “operable to” perform a task or tasks. In such contexts, “configured to,” “usable to,” and “operable to” are each used to connote structure by indicating that the devices/units/circuits/components include structure that performs the task or tasks during operation. As such, the device/unit/circuit/component can be said to be configured to, usable to, or usable to perform the task even when the specified device/unit/circuit/component is not currently operational (e.g., is not on or in operation). The devices/units/circuits/components used with the “configured to,” “usable to,” or “operable to” language may include electronic hardware—for example, circuits, memory storing program instructions executable to implement the operation, etc.—mechanical devices, or other types of structure. Reciting that a device/unit/circuit/component is “configured to,” “usable to,” or “operable to” perform one or more tasks is expressly intended not to invoke 35 U.S.C. § 112(f), for that device/unit/circuit/component.
In some embodiments, various items of information relating to seismic surveying may be embodied in a geophysical data product. A “geophysical data product” may be stored on a computer-readable, non-transitory medium and may embody geophysical data (such as raw streamer data, processed streamer data, two- or three-dimensional maps based on streamer data, etc.). Some non-limiting examples of computer-readable media may include hard drives, CDs, DVDs, print-outs, etc. In some embodiments, raw analog data from streamers may be stored as a geophysical data product. In other instances, the data may first be digitized and/or conditioned prior to being stored as the geophysical data product. In yet other instances, the data may be fully processed into a two- or three-dimensional map of the various geophysical structures before being stored in the geophysical data product. The geophysical data product may be produced offshore (e.g., by equipment on a vessel) or onshore (e.g., at a facility on land) either within the United States or in another country. If the geophysical data product is produced offshore or in another country, it may be imported onshore to a facility in the United States. Once onshore in the United States, geophysical analysis may be performed on the geophysical data product.